Using a field effect device for identifying translocating charge-tagged molecules in a nanopore sequencing device

ABSTRACT

A detector apparatus includes a field-effect transistor configured to undergo a change in amplitude of a source-to-drain current when at least a portion of a charge-tagged molecule translocates through the nanopore. In some implementations, the field-effect transistor is a carbon nanotube field effect transistor and the nanopore is located in a membrane. In other implementations, the field-effect transistor is a carbon nanotube field effect transistor and the nanopore is implemented in the form of a nano-channel in a semiconductor layer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional of U.S. application Ser. No.13/621,745 filed on Sep. 17, 2012, which in turn claims priority to U.S.Provisional Application 61/536,327 filed on Sep. 19, 2011, entitled“ssDNA Translocation Control and Nucleotide Sequencing Nano-DeviceMechanisms” and U.S. Provisional Application 61/536,381 filed on Sep.19, 2011, entitled “ssDNA Translocation Control and NucleotideSequencing Nano-Device Mechanisms”, all of which are incorporated hereinby reference in their entirety. The present application is also relatedto U.S. patent application Ser. No. 13/621,735 filed on Sep. 17, 2012,entitled “Translocation and Nucleotide Reading Mechanisms for SequencingNanodevices”, Attorney Docket No. P1075-US, and U.S. patent applicationSer. No. 14/576,148 filed on Dec. 18, 2014, entitled “Translocation andNucleotide Reading Mechanisms for Sequencing Nanodevices”, AttorneyDocket No. P1075-USD, which are also incorporated herein by reference intheir entirety.

FIELD

The present teachings relate to detection devices that may be used inconnection with charge-tagged molecules translocating through ananopore. More specifically, the present disclosure relates to detectiondevices that can be used for identification of nucleic acid sequences incharge-tagged DNA/RNA molecules.

BACKGROUND

Inexpensive and time-efficient full genome sequencing will enableprediction and impact-minimization of diseases through personalized,preventive medicine. Full genome sequencing is clearly of greatimportance for research into the basis of genetic disease. For example,access to a large database of individualized genome sequences willfacilitate cross-correlating gene-type to gene-function.

However, full-genome sequencing with current technologies (e.g.chemical- or enzymatic-based shot-gun DNA sequencing, includingmassively parallel automated sequencers based on slab gel separation orcapillary electrophoresis) is inadequate both in terms of performanceand also in terms of cost.

SUMMARY

According to a first aspect of the present disclosure, a detectorapparatus includes a membrane and a first field effect transistor. Themembrane contains a nanopore and the first field-effect transistor isconfigured to undergo a change in amplitude of a source-to-drain currentwhen at least a portion of a charge-tagged molecule translocates throughthe nanopore.

According to a second aspect of the present disclosure, a detectorapparatus includes a field-effect transistor and a solid-state membrane.The field-effect transistor has a planar gate terminal, and thesemiconductor layer has a nano-channel with a first portion of thenano-channel abutting a major surface of the planar gate terminal.

According to a third aspect of the present disclosure, a method ofdetection includes: a) forming a charge-tagged molecule, b) propagatingthe charge-tagged molecule through a nanopore of a membrane, and c)detecting a first amplitude of a source-to-drain current in a firstfield effect transistor, the first amplitude indicative of at least onecharge tag in the charge-tagged molecule modifying an electrostaticpotential of a gate portion of the first field-effect transistor.

Further aspects of the disclosure are shown in the specification,drawings and claims of the present application.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments of thepresent disclosure and, together with the description of a few exampleembodiments, serve to explain the principles and implementations of thedisclosure. The components in the drawings are not necessarily drawn toscale. Instead, emphasis is placed upon clearly illustrating variousprinciples. Moreover, in the drawings, like reference numerals designatecorresponding parts throughout the several views.

FIG. 1 shows a detection apparatus for detecting charge-tagged DNAmolecules propagating through nanopores of a membrane in accordance witha first embodiment of the present disclosure.

FIG. 2 shows some details of a carbon nanotube FET (CNTFET) that is apart of the detection apparatus in accordance with the presentdisclosure.

FIG. 3 shows the effect of changes in charge-tag magnitudes on adrain-to-source current-voltage (I-V) characteristic graph of a CNTFETin accordance with the present disclosure.

FIG. 4 shows a graph of electrostatic potential with and without thecharge tag contribution from a 10-base charge-tagged ssDNA oligomer atvarious distances from a gate portion of a CNTFET.

FIG. 5 shows a detection apparatus for detecting charge-tagged DNAmolecules propagating through a nanopore in accordance with a secondembodiment of the present disclosure.

FIG. 6 shows a detection apparatus for detecting charge-tagged DNAmolecules propagating through a nanopore in accordance with a thirdembodiment of the present disclosure.

FIG. 7 shows various configurations for mounting a pair of CNTFETs upona membrane in accordance with the present disclosure.

FIG. 8 shows additional elements that are coupled to a pair of CNTFETsfor carrying out various types of processing operations upon dataprovided by the pair of CNTFETs.

DETAILED DESCRIPTION

Throughout this description, embodiments and variations are describedfor the purpose of illustrating uses and implementations of theinventive concept. The illustrative description should be understood aspresenting examples of the inventive concept, rather than as limitingthe scope of the concept as disclosed herein.

The various embodiments described herein are generally directed at usingrealizable solid-state membranes, more specifically realizable siliconnitride semiconductor membranes having nanopores. One or more fieldeffect transistors (FETs) are located near one or more nanopores and theFETs are used to detect charge-tagged molecules that are propagatedthrough the nanopores by using an ionic current.

The FETs used for this purpose are sized and structured differently fromconventional FETs. More specifically, the source-to-drain current(I_(DS) current) in a conventional FET is typically controlled bychanging the dimension of a conducting channel located between thesource and drain terminals. This change in dimension is carried out byapplying a voltage to a gate region of the FET via a gate terminalprovided in the form of a metal contact. As can be understood, the sizeof the conducting channel is proportional to the length and the width ofthe gate region. The width may be selected to be much larger than thelength, in order to provide amplification of a signal applied to thesource terminal of the device. Furthermore, while the width limits thecurrent carrying capacity and electrostatics sensitivity of the device,the length of the gate region limits the upper switching frequency ofthe device.

In contrast to the sizing and structure of a conventional FET, the FETsused in accordance with the present disclosure are miniature sizeddevices achieved by using carbon nanotubes (CNTs).

Carbon nanotube FETs (CNTFETs) have been built and demonstrated byvarious entities. For example, attention is drawn to a paper by Wind etal (reference [5]), which describes a CNTFET incorporating top gateelectrodes. However, a CNTFET adapted for various embodiments inaccordance with this present disclosure contains a modified gate region.More specifically, a CNTFET in accordance with the present disclosure isstructured such that the source-drain current (I_(DS) current) iscontrolled by charges present external to the CNTFET in the vicinity ofthe gate region, thereby dispensing with the conventional gate terminal(metal contact for providing the gate voltage). The structural basis anduse of this CNTFET in accordance with the present disclosure will bedescribed below in more detail using the various figures.

Attention is first drawn to FIG. 1, which shows a detection apparatus105, which may be alternatively referred to herein as a nanoporesequencing device, for detecting charge-tagged molecules propagatingthrough nanopores 165 and 170 of a membrane 120 in accordance with afirst example embodiment of the present disclosure. Detection apparatus105 contains an upper reservoir 110 and a lower reservoir 115 with amembrane 120 disposed between the two reservoirs. Membrane 120 containsseveral nanopores (only two of which are shown in FIG. 1 forconvenience). Reservoirs 110 and 115 contain a solution, such as forexample, a mixture of water and potassium chloride (KCl), which isconducive to generating an ionic current upon application of a suitablevoltage via a voltage source 180.

When the ionic current is generated, charge-tagged species, such ascharge-tagged DNA molecules 145 and 175, propagate through nanopores 165and 170 respectively of the nanopore sequencing device.

Charge-tagged DNA molecules can be generated in a variety of ways. Forexample, oligonucleotide charge-tags may be implemented via solid-phasesynthesis using a phosphoramidite method (as disclosed in reference [4])and phosphoramidite building blocks derived from protected2′-deoxynucleosides (dA, dC, dG, and T), ribonucleosides (A, C, G, andU), or chemically modified nucleosides. Oligonucleotides between 16-28bases long may be obtained by sequentially coupling the building blocksto the growing oligonucleotide chain in the order required by thesequence of the product. In accordance with various embodiments of thepresent disclosure, only the net charge of the oligonucleotide is used.Consequently, it is desirable to minimize the radius of gyration andchoose a sequence based on partial complementarity along the chain so asto enable formation of looped structures with one exposed end having aminimum of 3-4 bases free for stable binding to a single-strand DNA(ssDNA) backbone. Having 3-4 bases free translates to a total of 64-256different oligonucleotide combinations. The product of the solid-statesynthesis is then released from the solid phase to a ssDNA solution,deprotected, and collected. The prepared oligonucleotides diffusivelybind in solution to complementary codons in the ssDNA/RNA to form thecharge tags.

Turning now to nanopores 165 and 170 contained in membrane 120, it maybe pertinent to point out that only nanopore 165 and associated elementshave been referred to in the description below. However, it should beunderstood, that the description is equally applicable to other similarnanopores and associated elements. (In this context, attention is drawnto jagged break 125 that is indicative of membrane 120 containing alarger area and more nanopores).

Nanopore 165 is populated with two CNTFETs 130 and 135 that are shownlocated inside membrane 120 and diametrically opposed to each other withrespect to nanopore 165. Each of the two CNTFETs 130 and 135 operate aslow dielectric constant FETs in a solvated environment (ionic solution).

A substantial portion of the gate of each of CNTFETs 130 and 135 isoriented parallel to a direction of flow of molecule 145 (indicated byarrow 140) and exposed to the nano channel. The amplitude of asource-to-drain current (I_(Ds) current) flowing in each of the twodevices is modified when molecule 145 is located in proximity to thegate portions. In one embodiment, the I_(DS) current increases when thetagged-charge present in molecule 145 is proximally located to the gateregion of either CNTFET. Further details pertaining to detectionapparatus 105 will be provided using other figures.

FIG. 2 shows additional details of CNTFET 130 that is shown in FIG. 1.It will be understood that in certain applications such as the one shownin FIG. 2, a single CNTFET 130, will be used, whereas in otherapplications CNTFET 130 will be used in conjunction with one or moreadditional CNTFETs such as CNTFET 135 shown in FIG. 1. The use ofmultiple CNTFETs provides various advantages such as, for example,increased detection sensitivity, redundant operation, and error checkingfeatures.

CNTFET 130 includes a gate portion 225 having a major surface 226arranged in parallel to the direction of propagation 140 of acharge-tagged molecule, which in this example, is a charge-taggedsingle-strand DNA (ssDNA) molecule 245. In other implementations, thecharge-tagged molecule may be other molecules, such as for example,charge-tagged RNA molecules. The charge-tagged ssDNA molecule 245 has a3 to 4 base oligomer-ssDNA complementarity that provides thermodynamicstability to the complex, particularly using locked nucleic acid (LNAsas described in references [6] and [7]) oligomers. Oligonucleotide 250includes a hairpin length 255 that determines the charge.

Gate portion 225 comprises a dielectric material such as a polysilicon,which is wrapped around a carbon nanotube portion 220. The CNTFET 130further includes a source portion 215, a source pad 205, a drain portion230, and a drain pad 235. Source-to-drain biasing may be provided byapplying a U_(DS) voltage (not shown) via terminals 210 and 240 that areconnected to source portion 215 and drain portion 230 respectively. TheUps voltage sets up a quiescent current flow between the source anddrain portions. This quiescent current is modified in amplitude whencharge-tagged single-strand DNA (ssDNA) 245 is in proximity to gateportion 225 when propagating through nanopore 165.

The proximity parameter is determined in part by selecting a suitablediameter 265 of the nanopore 165. In one example embodiment, diameter265 is selected not to exceed 3 nm. Diameter 265 may be alternativelyviewed as a contributory factor for a separation distance 260 betweenmajor surface 226 and charge-tagged single-strand DNA (ssDNA) 245 thatis translocating through nanopore 165. In one example embodiment,separation distance 260 is about 2 nm.

The fabrication of CNTFET 130 may be carried out via a number oftechniques. In one approach, semiconducting single-walled carbonnanotubes are used because such devices provide certain advantages (suchas faster switching properties at low source/drain voltages) overmetallic single-walled and metallic multi-walled tubes. Thesilicon-based fabrication of nanopores may be carried out via processessuch as described in reference [1]. The fabrication process described inreference [1] may be modified to include additional steps that includeincorporating CNTFET 130 within or outside membrane 120.

In general, the fabrication process may begin with building awrap-around CNTFET (preferred over top-gated devices like the onedisclosed in reference [5]) since the wrap-around CNTFET provides animproved device on/off ratio. The wrapping is carried out using adielectric material that is wrapped around the entire length of theCNTFET. The CNTFET is then placed upon membrane 120 and a portion of thewrapping is removed (for example, by etching) so as to expose theextremities of carbon nanotube portion 220. Source, drain, and gatecontacts may then be provided. A focused electron beam may be used tocreate one or more 2-3 nm nanopores in membrane 120. The nanoporefabrication process may be carried out as disclosed in reference [1].Then one or more CNTFETs are incorporated into or on membrane 120. Inthe FIG. 1 embodiment, the nanopore is created between the two CNTFETs.

Attention is now drawn to FIG. 3, which shows the effect of changes incharge-tag magnitudes on a drain-to-source current-voltage (I-V)characteristic of a CNTFET in accordance with the present disclosure.Each of the curves represent gate-to-source voltages (U_(GS))corresponding to various gate charges (q-). Using curve 305 for purposesof explanation, it can be observed that curve 305 includes asubstantially linear rising portion 310 (where I_(DS) risessubstantially linearly in correspondence to an increase in U_(DS) thedrain-to-source voltage); and a substantially flat portion 315 (wherethe I_(DS) remains substantially constant upon increase in U_(DS)). Thelinear rising portion 310 corresponds to a linear mode of operation ofdetection apparatus 105 and it is this linear mode of operation that isused in the various embodiments in accordance with the presentdisclosure. The substantially flat portion 315, which corresponds to asaturated region of operation, is not typically used when identifyingtranslocating charge-tagged molecules.

The linear mode of operation is used so as to enable identification anddetection of the 64-256 different free end oligonucleotide combinationsthat correspond to the 3-4 bases which are free to bind stably to abackbone complementary sequence of bases of a ssDNA molecule. Each ofthe 64-256 combinations generates a uniquely identifiable change inI_(DS) current in a CNTFET. In contrast, the saturated mode of operationwould be unsuitable for use in detecting these various combinationsbecause I_(DS) would remain relatively constant for the variouscombinations. However, the saturated region of operation may be utilizedin various other applications in accordance with the disclosure thatsupport a CNTFET on-off switching mode of operation.

FIG. 4 shows a graph of electrostatic potential versus charge tagcontribution from a 10-base oligomer charge-tagged ssDNA at variousdistances (“r”) from a CNTFET gate.

Curve 410 indicates the total electrostatic potential without acharge-tag and curve 405 indicates the changed electrostatic potentialdue to the presence of a 10-base oligonucleotide charge-tag as afunction of varying distances (r) from a CNTFET gate. At r=1 nm thecharge tag is a few tenths of a Volt, sufficient to activate I_(DS)current change in the CNTFET.

A predictive simulation involving a Non-Equilibrium Green's Function(NEGF) approach was used to compute the current-voltage (IV) and deviceconductance graph shown in FIG. 4. Carbon atoms were described using anextended Huckel model (as disclosed in reference [2]) with Cerda basis(reference [3]). For a 1.8 nm gate: a V_(GD)=−0.4V increases thenanotube resistance from 22.13-22.14 kΩ, and for a 0.9 nm gateV_(GD)=−0.4V increases nanotube resistance from 22.14-22.20 kΩ. Largergating effect would lead to higher nucleic acid distinguishability in anucleobase sequencing nanopore device.

In one quantum mechanical simulation using an explicit solvent model,charge-screening effects on the gate potential were predicted usingshort (−10 bases long) 3-base complementary oligomeric charge tags onspecific ssDNA sequences. In this predictive quantum mechanicalcalculation, the detector apparatus remains operational within r<=−1 nm,and provides discriminatory electron current signatures I_(SD) fordifferent nucleic acid sequences with charge-tags differing by 2-4e.

FIG. 5 shows a detection apparatus 500 for detecting charge-tagged DNAmolecules propagating through a nanopore in accordance with a secondembodiment of the present disclosure. In this embodiment, CNTFET 535 hasa planar configuration that may be fabricated using semiconductorfabrication techniques. CNTFET 535 includes a carbon nanotube portion520, a dielectric portion 515, a source portion 525, a drain portion530, and a gate portion 510 having a major surface 511 abutting achannel 505. Channel 505 is representative of a nanopore through which acharge-tagged molecule, such as charge-tagged single-strand DNA (ssDNA)245, is translocated using an ionic solution. Channel 505 may befabricated in the form of a groove in a semiconductor layer 540, with alongitudinal axis of the groove projecting out of the plane of the paperon which FIG. 5 is drawn. It will therefore be understood thatcharge-tagged single-strand DNA (ssDNA) 245 translocates along thislongitudinal axis, or in other words in a direction that may be viewedas emerging out of the drawing sheet on which FIG. 5 is drawn.

Furthermore, it will be understood that though channel 505 is shown inFIG. 5 as having a rectangular cross-section, various othercross-sectional configurations such as a circular cross-section, may beused instead. Generally, the cross-section is selected so as to providean interaction region with gate 510 whereby a charge-tagged moleculepropagating through the nanopore has an effect on the I_(DS) currentflow in CNTFET 535.

In a sixth configuration 730, four CNT FETs are used. Specifically, afirst CNTFET 130 and a second CNTFET 731 are each located on opposingmajor surfaces 706 and 707 on one side of nanopore 165. A third CNTFET135 and a fourth CNTFET 732 are each located on opposing major surfaces706 and 707 on a diametrically opposite side of nanopore 165.

In contrast, to the embodiment shown in FIG. 5 (wherein CNTFET 535 isused as a single detector for identifying charge-tagged moleculespropagating through a nanopore), the third embodiment of a detectorapparatus 600 shown in FIG. 6 incorporates two detectors.

The first detector is a CNTFET 535 that has been described above. Thesecond detector is implemented in the form of a graphene nanoribbonlayer 605 that abuts an opposing surface 606 of channel 505. Opposingsurface 606 opposes major surface 511 described above. A quiescentcurrent may be generated across the graphene nanoribbon 605 uponapplication of a suitable biasing voltage between the two ends laid ontop of the silicon support 605 by using a suitable voltage source (notshown). This quiescent current is affected when a charge-tagged moleculepropagates through nanopore 505. The change in current, which may bereferred to herein as a sense current, is used to detect the variousfree end oligonucleotide combinations that may be present in a backboneof a charge-tagged ssDNA molecule propagating through nanopore 505.

FIG. 7 shows various configurations for mounting a pair of CNTFETs upona membrane in accordance with the present disclosure. In a firstconfiguration 705, a first CNTFET 130 is located upon a first majorsurface 706 of membrane 120 on one side of nanopore 165. A second CNTFET135 is located upon the same first major surface 706 of membrane 120 buton a diametrically opposite side of nanopore 165.

In a second configuration 710, the first CNTFET 130 is located upon anopposing major surface 707 of membrane 120 on one side of nanopore 165.A second CNTFET 135 is located upon the same opposing major surface 707of membrane 120 but on a diametrically opposite side of nanopore 165.

In a third configuration 715, the first CNTFET 130 is located upon afirst major surface 706 of membrane 120 on one side of nanopore 165. Asecond CNTFET 135 is located upon the opposing major surface 707 ofmembrane 120 on a diametrically opposite side of nanopore 165.

In a fourth configuration 720, the second CNTFET 135 is located upon afirst major surface 706 of membrane 120 on one side of nanopore 165. Thefirst CNTFET 130 is located upon the opposing major surface 707 ofmembrane 120 on a diametrically opposite side of nanopore 165.

In a fifth configuration 725, one or more CNTFETs may be embedded insidemembrane 120. (FIG. 1 shows several CNTFETs embedded in membrane 120).In one embodiment wherein one or more CNTFETs are embedded insidemembrane 120, membrane 120 is selected to have a thickness 726 between20-50 nanometers.

FIG. 8 shows additional elements that are coupled to a pair of CNTFETsfor carrying out various types of processing operations upon dataprovided by the pair of CNTFETs. More particularly, the I_(DS) currentsgenerated in CNTFETs 130 and 135, which may be a part of currentdetectors 830 and 835 respectively, are amplified in instrumentationamplifiers 805 and 810 for generating amplified voltage signals. Thevoltage signals generated in the two amplifiers are provided to a pairof analog-to-digital converters (ADCs) 810 and 820. The digital signalsgenerated by the pair of converters from the amplified voltage signalsare provided to processing circuit 825, which processes the digitalsignals to obtain meaningful information. Processing circuit 825includes various elements (not shown) such as a processor, acomputer-readable storage medium for storing data and/orsoftware/firmware programs, and input/output interface elements.

It will be understood that the digital data provided to processingcircuit 825 by ADCs 810 and 820, may be used for implementing variousfeatures in accordance with the present disclosure, such as redundantoperations, error-correction operations, and/or parallel processingoperations. For example, in a redundant mode of operation, a failure inone of the two CNTFETs may be addressed by ignoring the failed CNTFETand using only the one operational CNFT. In an error-correction mode ofoperation the digital data provided by the two CNTFETs may be comparedagainst each other in order to ensure accuracy and robustness ofoperation. In parallel processing operations, the I_(DS) currentsgenerated by two or more CNTFETs 130 and 135 may be used to derive avariety of information pertaining to a single charge-tagged molecule, ormultiple charge-tagged molecules that may be concurrently orsequentially propagating through a nanopore.

Furthermore, while FIG. 8, shows CNTFETs 130 and 135 in a detectionconfiguration corresponding to the first embodiment described above, theprocessing operations and elements described using FIG. 8 are equallyvalid for various other embodiments (for example, the embodiment shownin FIG. 6) wherein a pair of CNTFETs are employed.

Turning away from FIG. 8, attention is once again drawn to FIG. 2. Theinteraction between CNTFET 130 and charge-tagged single-strand DNA(ssDNA) 245 may be calculated by assuming a minimized gate capacitance(i.e. A and ε₀ ε_(r) in the following equation for gate capacitance:C=ε₀ ε_(r)A/r, where “r” is the separation distance 260); an estimatedCoulomb interaction effect (for example, from water) via expressing thecontribution to the E-field at a point in space (CNTFET gate surface)due to a single, discrete point charge (ne, from charge species) locatedat another point in space, distanced by r, results in an electricalpotential of >=0.9V for r<=2 nm. This calculation also assumes: discretewater molecules in the nanopore have a dielectric constant dominated bytheir orientation (large dipole moment), stronger screening effects in(K+) ionic solution (the negatively charged ssDNA backbone and negativecharge-tags attract positive counter ions), and that the complex appearsneutral from distance larger than the backbone-counter ion distance(although the nanopore may strip ions from ssDNA backbone duringtranslocation) to determine a worst case scenario for operating theCNTFET device in the linear mode of operation.

A test model comprising a nanotube (m=5, n=0, length=4.3 nm), left andright metallic electrodes, a dielectric region (with dielectric constant4) separating the nanotube and the gate, and a gate bias voltage wasused to assess performance of a modeled embodiment of the presentdisclosure. The gate size was selected to be approximately 1.2×1.2 nm,which approximates to the electric field of a point charge. Assuming acharge-tag with 20e (i.e. 20 nucleotides long), a small nanopore(1.5-2.5 nm) capable of stripping counter ions from the charge-tag, anda distance of approximately 1 nm between the charge-tag and a CNTFETgate insulator surface leads to a potential at the insulator of −0.4V(screened Coulomb).

In conclusion, a detector apparatus in accordance with the presentdisclosure enables sufficiently small charge tags (for example, 8-136bases, considering a 3-base complementary overlap between charge tag andssDNA) on ssDNA to gate-activate a CNTFET in a linear mode of operation.The detector apparatus preferably has a device sensitivity that enablesdetection of small tags with 2-4e potential difference at distances ˜1nm in a solvated environment (ionic solution described above).

All patents and publications mentioned in the specification may beindicative of the levels of skill of those skilled in the art to whichthe disclosure pertains. All references cited in this disclosure areincorporated by reference to the same extent as if each reference hadbeen incorporated by reference in its entirety individually.

It is to be understood that the disclosure is not limited to particularmethods or systems, which can, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. As used in this specification and the appended claims, thesingular forms “a,” “an,” and “the” include plural referents unless thecontent clearly dictates otherwise. The term “plurality” includes two ormore referents unless the content clearly dictates otherwise. Unlessdefined otherwise, all technical and scientific terms used herein havethe same meaning as commonly understood by one of ordinary skill in theart to which the disclosure pertains.

The examples set forth above are provided to give those of ordinaryskill in the art a complete disclosure and description of how to makeand use the embodiments of the terrain traversal device of thedisclosure, and are not intended to limit the scope of what theinventors regard as their disclosure. Modifications of theabove-described modes for carrying out the disclosure may be used bypersons of skill in the robotic arts, and are intended to be within thescope of the following claims.

A number of embodiments of the disclosure have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the presentdisclosure. Accordingly, other embodiments are within the scope of thefollowing claims.

LIST OF REFERENCES

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3. Cerda, J. and F. Soria, Accurate and transferable extendedHuckel-type tight-binding parameters. Physical Review B, 2000. 61(12):p. 7965-7971.

4. Beaucage, S. L. and M. H. Caruthers, DeoxynucleosidePhosphoramidites—a New Class of Key Intermediates forDeoxypolynucleotide Synthesis. Tetrahedron Letters, 1981. 22(20): p.1859-1862.

5. Wind, S. J., et al., Vertical scaling of carbon nanotube field-effecttransistors using top gate electrodes (vol 80, pg 3817, 2002). AppliedPhysics Letters, 2002. 81(7): p. 1359-1359.

6. Singh, S .K., et al., LNA (locked nucleic acids): synthesis andhigh-affinity nucleic acid recognition. Chemical Communications,1998(4): p. 455-456.

7. Koshkin, A. A., et al., LNA (Locked Nucleic Acids): Synthesis of theadenine, cytosine, guanine, 5-methylcytosine, thymine and uracilbicyclonucleoside monomers, oligomerisation, and unprecedented nucleicacid recognition. Tetrahedron, 1998. 54(14): p. 3607-3630.

What is claimed is:
 1. A detector apparatus comprising: a field-effecttransistor having a planar gate terminal; and a semiconductor layercomprising a nano-channel with a first portion of the nano-channelabutting a major surface of the planar gate terminal.
 2. The apparatusof claim 1, wherein the nano-channel is configured to transport at leastone charge-tagged molecule along a longitudinal axis that traverses themajor surface of the planar gate terminal, and further wherein thefield-effect transistor is a carbon nanotube field effect transistorconfigured to undergo a change in amplitude of a source-to-drain currentwhen the at least one charge-tagged molecule traverses the major surfaceof the planar gate terminal.
 3. The apparatus of claim 2, wherein thenano-channel has a circular cross-section and is operative as a nanoporehaving a diameter selected to provide a separation distance notexceeding about 2 nm between the at least one charge-tagged molecule andthe major surface of the planar gate terminal.
 4. The apparatus of claim3, wherein the diameter does not exceed about 3 nm.
 5. The apparatus ofclaim 3, further comprising a graphene nanoribbon located on a majorsurface of the semiconductor layer with a portion of the graphenenanoribbon covering an exposed portion of the nano-channel.
 6. Theapparatus of claim 5, wherein the graphene nanoribbon is configured as acurrent detector to detect the at least one charge-tagged molecule whenthe at least one charge-tagged molecule traverses the major surface ofthe planar gate terminal.
 7. The apparatus of claim 6, whereinconfiguring the graphene nanoribbon as the current detector comprisesconfiguring the graphene nanoribbon to measure a sense current flowingthrough at least a portion of the graphene nanoribbon.